Heretofore, as an electrophotographic developer used to make an electrostatic latent image formed on an electrophotographic light-sensitive layer visible, a one-component developer or a two-component developer has been used. The one-component developer is prepared by melt kneading a mixture of a resin (e.g, polystyrene, a styrene-butadiene copolymer and a polyester) and a pigment or dye (e.g, carbon black and Phthalocyanine Blue) as a colorant, and then grinding it. The two-component developer comprises a toner and a carrier having, for example, an average particle diameter nearly equal to that of the toner or up to 500 .mu.m, and the carrier is a glass bead, a particle of iron, nickel or ferrite, or those covered with a resin.
These developers, when used without other additives, are not satisfactory in storage stability (antiblocking), conveying properties, developability, transferability, charging properties, and so forth. Thus, in order to improve these properties, additives are often added. Hydrophobic fine powders are used as an additive, such as hydrophobic silica, a mixture of silica fine particles and alumina or titania fine particles, alumina-covered titania fine particles, and so forth. As the titania, titania having a rutile or anatase crystal structure is used.
By using hydrophobic fine powders such as silica fine particles now often used, the properties of the developers are considerably improved with respect to storage stability, conveying properties, developability and transferability. However, if they are used in such an amount that the above properties are sufficiently improved, a problem arises in that chargeability is adversely influenced. Concerning chargeability, the developers are required to exhibit satisfactory efficiency with respect to charged amount, rapid charging ability, distribution of charged amount, admixing properties, charging stability under various atmosphere, and so forth When silica fine particles are used for example, they exert adverse influences on the rapid charging ability, the distribution of charged amount, the admixing properties, and the charging stability.
Addition of alumina or titania fine particles together with silica fine particles as admixture improves the rapid charging ability, the distribution of charged amount, the admixing properties and the charging stability, but it results in marked decrease in the charged amount. All the above-mentioned requirements for chargeability are met using the admixture only under specific conditions, and the improving effects are not satisfactory, particularly in the charging stability under various atmosphere.
Rutile-type or anatase-type titania fine particles to be used as an additive are necessarily subjected to treatments, such as a treatment for making the particles hydrophobic by using various coupling agents and a treatment for coating the particles with alumina. Otherwise, the untreated titania particles are hardly charged. Titania fine particles subjected to the hydrophobic treatment using a coupling agent are effective for improving the chargeability to some extents but the effect is still insufficient. In particular, satisfactory charging stability cannot be attained when used with toners comprising a polyester as a binder resin. On the other hand, the alumina treatment does not effectively prevent aggregation of the titania particles and the titania particles exhibit poor dispersibility.